Magneto-electronic devices such as magnetoresistive random access memory (MRAM) cells, magnetic sensors, read-heads, and the like have become increasingly popular in recent years due to the large signal available from recently-developed magnetoresistive materials. MRAM has the advantages of nonvolatile storage, radiation resistance, fast read and write operations, and much better endurance than other nonvolatile memories. Such devices typically incorporate a magnetic tunnel junction (MTJ) structure (or “stack”) that includes multiple ferromagnetic layers separated by one or more non-magnetic layers. A typical MTJ stack might include two synthetic anti-ferromagnets (SAFs)—a free-layer SAF, and a pinned SAF.
Such SAFs are temperature dependant. That is, their magnetic properties are strongly dependent upon the ambient thermal environment which limits the range of temperatures at which the device may operate. For example, the antiferromagnetic coupling strength, typically represented by the saturation field, Hsat, of a NiFe SAF measured at temperature typically drops, as temperature is increased, at a rate of about 0.4%/° C. The saturation field, Hsat is defined as the field needed to align the moments of the two ferromagnetic layers in a SAF parallel to each other. This drop, though reversible, leads to a reduced operating window at elevated temperature as the Hsat is an important parameter determining both the minimum switching field of the bit and the upper limit of the operating range of the bit.
The uniaxial anisotropy of the material, Hk also affects the switching field of the bit and the size of the operating window. Hence the SAF material must be chosen also for the optimum uniaxial anisotropy. For MRAM devices with significant shape anisotropy, it is desirable to minimize the anisotropy of the material to keep the switching field low and the operating window large. The uniaxial anisotropy of the material is expressed as the field needed to saturate the magnetic moment of that material along the hard axis.
SAFs also can contribute to an increase in device resistance resulting from high processing temperatures or long times at operating temperatures. The increase in resistance is primarily due to thermally-activated oxidation of SAF material around the edges of the patterned bits. The oxidation encroachment leads to an increase in the resistance of the MTJ as the bit borders become non-conducting, effectively reducing the area of the MTJ; this effect causes a larger relative resistance increase in smaller MTJ bits. Hence, reducing oxygen encroachment effects would lead to less increase in the resistance of the patterned bits, smaller differences between different size bits (improved scaling) and allowing higher temperature treatments.
Accordingly, it is desirable to provide a MTJ stack with better high-temperature operation and reduced resistance scaling from oxygen encroachment. Maintaining a low uniaxial anisotropy of the ferromagnetic layers is typically desirable. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.